The present invention relates to cranes, more particularly to control of cranes for transferring cargo at sea so as to manage or counteract pendulation of suspended payloads.
Cranes have been used in diverse settings to effect lift-on, lift-off transfer of cargo. Various single-jib (single-boom) crane systems, both active and passive, have been considered and/or demonstrated for transferring cargo. A prevalent variety of single-jib crane is a slewing pedestal crane (also known as a rotary boom crane, or a rotary jib crane, or a luffing jib crane), which involves the suspension of a payload (load), via a hoist line (e.g., including one or more cables), from the tip of a rotatable boom (rotatable jib). Herein the terms “jib” and “boom”, are used interchangeably, and the terms “load” and “payload” are used interchangeably.
Conventional methods, devices, and algorithms for controlling slewing pedestal cranes are usually designed to avoid or minimize a fundamental problem associated with such control, namely, pendulation, which is the swinging or swaying of the payload attached to the hoist line. Pendulation generally represents a hindrance to crane operations, and tends to be exacerbated or intensified when the cargo transfer takes place in a marine environment. For instance, unmitigated pendulation that is caused by seaway disturbances to the marine vessel (e.g., ship or barge) upon which a crane is mounted may prevent the accurate placement of containers onto boats (e.g., lighters) for transport to shore.
A hoist line, together with its attached and suspended payload, constitutes a pendulum characterized by an oscillation period that may be responsive, to the point of resonance, with seaway-induced motion of the ship. This inclination toward resonance may increase with increasing length of the hoisting line, which may tend to lengthen in accordance with horizontally closer positioning of the payload to the pedestal. Generally speaking, pendulation of a crane system utilized at sea can be suppressed by (i) alleviating the ship motion (e.g., by removing or otherwise affecting the mechanism causing the ship motion), and/or (ii) altering the dynamic response of the crane system to the ship motion.
A simple type of slewing pedestal crane includes a jib (boom) and a payload hoist line. The payload hoist line extends between the tip of the jib (boom) and the payload. Control of the crane is effected in three degrees-of-freedom, viz., slew (horizontal rotational motion of the boom that results in translation of the payload in a direction transverse to the orientation of the jib), luff (vertical rotational motion of the boom that results in translation of the payload in a direction parallel to the orientation of the jib), and hoist (vertical translation of the payload).
More complicated than the simple type of slewing pedestal crane is an RBTS-equipped crane, a type of slewing pedestal crane that incorporates a Rider Block Tagline System. In basic principle, the RBTS seeks to reduce pendulation by using a rider block to reduce the length of the pendulum. The shortened pendulum has shorter oscillation periods than would the pendulum in the absence of the rider block. In effect, the RBTS thereby “detunes” the pendulum from the ship motions, which have longer oscillation periods than does the shortened pendulum.
An RBTS-equipped slewing pedestal crane includes a jib (boom), a rider block (which is situated generally intermediate the boom tip and the payload), a rider block lift line (which is attached to the rider block and extends between the boom tip and the rider block), a payload hoist line (which is reeved through the rider block and extends between the jib tip and the payload), a left tagline beam, a right tagline beam, a left tagline (which is attached to the rider block and extends between the left tagline beam end and the rider block), and a right tagline (which is attached to the rider block and extends between the right tagline beam end and the rider block). An RBTS-equipped crane is characterized by the three aforementioned degrees of freedom (slew, lull, and hoist), plus two additional degrees of freedom, viz., the vertical and horizontal positions of the rider block.
The following United States patents, each of which is incorporated herein by reference, disclose various electro-mechanical and/or algorithmic approaches to assisting a crane operator in controlling a slewing pedestal crane: Agostini et al. U.S. Pat. No. 7,367,464 B1 issued 6 May 2008, entitled Pendulation Control System with Active Rider Block Tagline System for Shipboard Cranes”; Nayfeh et al. U.S. Pat. No. 6,631,300 B1 issued 7 Oct. 2003, entitled “Nonlinear Active Control of Dynamical Systems”; Naud et al. U.S. Pat. No. 6,505,574 B1 issued 14 Jan. 2003, entitled “Vertical Motion Compensation for a Crane's Load”; Robinett, III et al. U.S. Pat. No. 6,496,765 B1 issued 17 Dec. 2002, entitled “Control System and Method for Payload Control in Mobile Platform Cranes”; Jacoff et al. U.S. Pat. No. 6,444,486 B2 issued 11 Nov. 2003, entitled “System for Stabilizing and Controlling a Hoisted Load”; Jacoff et al. U.S. Pat. No. 6,439,407 B1 issued 27 Aug. 2002, entitled “System for Stabilizing and Controlling a Hoisted Load”; Robinett, III et al. U.S. Pat. No. 6,442,439 B1 issued 27 Aug. 2002, entitled “Pendulation Control System and Method for Rotary Boom Cranes”; Naud et al. U.S. Pat. No. 6,039,193 issued 21 Mar. 2000, entitled “Integrated and Automated Control of a Crane's Rider Block Tagline System”; Overton et al. U.S. Pat. No. 5,961,563 issued 5 Oct. 1999, entitled “Anti-Sway Control for Rotating Boom Cranes”; Robinett, III et al. U.S. Pat. No. 5,908,122 issued 1 Jun. 1999, entitled “Sway Control Method and System for Rotary Boom Cranes”; Nachman et al. U.S. Pat. No. 5,089,972 issued 18 Feb. 1992, entitled “Moored Ship Motion Determination System.” See also the following papers, incorporated herein by reference: Michael J. Agostini, Gordon G. Parker, Kenneth Groom, Hanspeter Schaub and Rush D. Robinett, “Command Shaping and Closed-Loop Control Interactions for a Ship Crane,” Proceedings of the American Control Conference, Anchorage, Ak., 8-10 May 2002, pages 2298-2304; Gordon G. Parker, Michael Graziano, Frank A. Leban, Jeffrey Green, and J. Dexter Bird, III, “Reducing Crane Payload Swing Using a Rider Block Tagline Control System,” Oceans 2007, Aberdeen, Scotland, 18-21 Jun. 2007 (5 pages).
For many crane applications, a slewing pedestal crane is favored because of its considerable lifting capacity and versatility, as it is capable of handling containerized cargo as well as vehicles and other outsized objects (e.g., ramps used for discharging vehicles at a pier). Nevertheless, a single-jib crane—even a slewing pedestal crane—has its limitations in terms of size, shape, and/or weight of the load being lifted. Among crane artisans there is recognition of the basic notion that some larger (more substantial/extensive/cumbersome) loads that are difficult to handle using one crane could possibly be better accommodated by combining the efforts of two or more cranes. However, the implementation of plural cranes to lift larger loads is easier said than done, especially in marine environments.
The literature is not abundant on the subject of cargo handling using a plurality of cranes or crane-like devices. Coordinated robotic maneuvers in the absence of base motion (i.e., assuming a stationary base) are disclosed by R. Smith, G. Starr, R. Lumia, and J. Wood, “Preshaped Trajectories for Residual Vibration Suppression in Payloads Suspended from Multiple Robot Manipulators,” Proceedings of the 2004 IEEE International Conference on Robotics & Automation (ICRA), New Orleans, La., 26 Apr.-1 May 2004, volume 2, pages 1599-1603, incorporated herein by reference. R. Smith et al. disclose an approach for developing swing-free motion trajectories for a dual-arm manipulator, but only in the context of a manufacturing environment, where base motion disturbances are not present.
The AutoLog (Automated Logistics) cargo handling system, recently under development by the U.S. Navy, is designed to suspend a payload from four cables. Each cable has associated therewith a computer-controlled winch, and extends from a jib supported by a fixed vertical mast. The long term goal of the AutoLog is to be capable of operating successfully in a high-sea-state environment.
The use of plural (e.g., several) cranes together to lift heavy or unwieldy loads is a recognized but rather uncommon practice. These “team lifts” are performed manually, and require the coordinated efforts of plural (e.g., several) individual operators. With respect to shipboard cranes, such team-lift operations have been successfully conducted with experienced operators and in very benign environmental conditions, but would not be attempted when significant ship motions are present.